Method of making selenium rectifiers



INVENTOR ARrHz/R Vow H/PPa A77 ENE) i! M TT' A. VON HIPPEL Filed April 13, 1942 METHOD OF MAKING SELENIUM RECTIFIERS Dec. 24, 1946.

Patented Dec. 24, 1946 METHOD OF MAKING SELENIUM RECTIFIERS Arthur von Hippel, Weston, Mass, assignor to Federal Telephone & Radio Corporation, New York, N. Y., a corporation of Delaware Application April 13, 1942, Serial No. 438,721

13 Claims. 1

This invention relates to selenium elements and particularly to elements of the type useful in rectifiers or other selenium cells.

The object of this invention is to provide a method for rapidly making such selenium elements.

A selenium element or plate commonly comprises a metal base plate on which the selenium is coated. Such selenium elements have heretofore been made by fusing a coating of powdered amorphous selenium on the base plate and then heating the plate for a long time such as a number of hours at a moderate temperature while maintaining pressure against the selenium surface of the disc; following which there would be a second heat treatment for a long time at a higher temperature. In the course of these heat treatments the selenium would change from the amorphous to the crystalline form. This former process has the great disadvantage of requiring such a great length of time to produce a satisfactory selenium element.

In accordance with my invention I am able to reduce the time of the 'heat treatment of the disc to a relatively short time, from a matter of hours to a matter of minutes. I accomplish this by giving the selenium coated plate a short initial heat treatment at a moderately high temperature and preferably without pressure, and then a short additional treatment, under pressure, at a much higher temperature, but below the melting point of the selenium. My theory is that the short initial heat treatment at a properly selected temperature causes numerous crystal nuclei to form rather evenly throughout the selenium coating and that the subsequent increase in the temperature under pressure causes these nuclei to grow rapidly and extend throughout the coating in such a way as to present a smooth crystalline selenium surface which is desired for the selenium element.

In the drawing Fig. l is a face view and Fig. 2 a cross sectional side view of a selenium coated disc adapted to be treated in accordance with the process of my invention; and Figs. 3 and 4 illustrate a side and top view, respectively, of a press adapted to be used in the application of. heat and pressure in the practice of my invention.

In Figs. 1 and 2 the base plate I is a disc which may be made in a well known manner of a material such as steel, and is ordinarily provided with a central hole H for the purpose of assembling on a suitable mandrel. The disc may have its upper surface roughened as by sand blasting to aid in holding the selenium. This holding surface preferably has applied to it a thin coating l2 of nickel, in a well known manner. The selenium layer l3 may be applied by depositing amorphous selenium powder on the base plate which is heated to a temperature above the melting point of the selenium, so that the selenium melts and flows and fuses to the nickeled surface of the disc. The selenium coated disc may then be quenched in a well known manner as by placing it on a surface maintained at about 20 C. For clearness, the layers 12 and I 3 are shown somewhat thickerthan they will usually be used in practice.

Broadly, my method of processing the selenium coated base plate is to heat it to a moderate temperature of preferably around C. for a short time, for example five minutes, to generate a sufiicient number of uniformly distributed crystal nuclei. This step is preferably performed without the application of pressure, as it is desired that the selenium shall not be caused to flow when it is heated. If any pressure should be applied, it should not be so great as to cause the undesired flow of selenium. The temperature need not be kept exactly at 100 0., although a temperature at about this value has been found most satisfactory. A temperature might, for example, be selected from within the approximate limits of 80 C. to C. Following this short initial heat treatment, I apply a high pressure, for example around 750 pounds per square inch at a high temperature somewhat below the melting point of the selenium, for a short time of a few minutes or even only a few seconds. The temperature used is preferably around 190 C., although both the pressure and the temperature may depart considerably from the preferred values. Changes in the preferred .temperature and pressure might require a longer period of time of the treatment. A permissible range within which the temperature might be selected for this pressure-heat treatment could be from around C. to about 218 C. and the permissible pressure could be anywhere from about 750 pounds per square inch down to around 250 pounds per square inch or even lower. When applying the pressure to the selenium. the pressure plates actually placed against the selenium should be of some material which does not injure the selenium when brought into contact with it, such as for example mica or aluminum sheets.

The following are two detailed examples of my invention within the scope of the broad v the press.

method above described, these examples having been found satisfactory in practice. According to the first of these I heat the selenium coated plate at 100 C. for five minutes without pressure. I then place the disc in a press wherein the temperature and pressure may be raised in steps. .A type of press which might advantageously be used for this Pu ose is shown by way of example in Figs. 3 and 4. This press may conveniently comprise the base member l4 which is provided with a heater cavity IS in which an electrical heating element may be placed and a thermometer well l6 in which a thermometer may be inserted to measure the temperature of A press plate l'l, provided with a central hole 3, may be placed on top of the base member. The upper pressure plate I9 is provided with a disc shaped cutout 20 adapted to fit over the disc shaped plate I1 and with a centrally located pin 2| adapted to fit into the hole I8. In the use of this press, the selenium coated disc 22 is placed on plate H with the selenium surface up; and over the selenium there is placed a disc 23 of a suitable material which does not injure the selenium such as mica or aluminum, then another thin disc 24 such as metal and a disc 25 of a suitable sponge like resilient material such as rubber.

After putting the selenium coated disc 22 in this press, I apply a pressure of about 250 pounds per square inch for about one-half minute during which time I cause the temperature to rise from 100 C. to 105 C. Then I raise the pressure to 500 pounds per square inch for the next one-half minute during which time I cause the temperature to rise from 105 C. to 120 C. Then I apply 750 pounds per square inch for the next three and one-half minutes during which time I cause the temperature to rise to 195 C. Then I release the pressure and raise the temperature to between 214 C. and 218 C. for the next fifteen minutes. Thereupon the disc is allowed to cool.

The second example of the process which I have found satisfactory is as follows. I maintain the selenium coated disc at the temperature of approximately 100 C. without pressure for several minutes. Then I raise the temperature to about 110 C. in about one and one-half minutes still without the application of any pressure. Then I'apply a pressure of about 240 pounds per square inch for the next one and one-. half minutes during which time I cause the temperature to rise to about 120 C. Then I increase the pressure to about 720 pounds per square inch for the next minute during which time I cause the temperature to rise to about 130 C. I cause the temperature to continue to rise still with the application of the 720 pound pressure until the end of sixteen minutes from the beginning of the heat treatment. Then I release the pressure and raise the temperature further to about 210 C., keeping it there for about fourteen more minutes. Then I take the plate out and allow it to cool.

Following a heat and pressure treatment as described above, the selenium disc is ready to have a counter-electrode material applied to the surface of the selenium in a well-known manner as by coating it with a suitable conducting metal which makes a close contact with the selenium. The selenium disc may then be electroformed in a well-known manner by applying voltage between the base plate In and-the counter elecro e.

By processing the selenium discs in accordance with my invention, the total time of the heat treatment is reduced to only a few minutes, and selenium discs are provided which are satisfactory in every way for use in rectifiers.

Another advantage of the heat treating method according to my invention is that it will enable selenium discs to be made by a continuous process in an assembly line wherein the discs may be coated with selenium in the initial stage of the assembly line, then have the heat and pressure treatment in a heated press in a subsequent stage, following which the counter electrode may be sprayed on.

The foregoing procedure is given as one which has been found satisfactory. It should be understood, however, that the procedure might be departed from in some respects, while still obtaining its advantage of a short time of treatment. For example, the times of maintaining the various temperatures and pressures need not be exactly as outlined above, but may be departed from to a reasonable extent. The invention is not limited except as defined by the appended claims.

What is claimed is: g

1. The method of making a selenium plate which comprises coating the plate with a, layer of"amorphous selenium, maintaining the temperature of the coated plate at about 100 C. for a brief time without the application of substantial pressure and then raising the temperature to about 190 C. while pressure is applied against the surface of the selenium.

2. The method of processing a selenium coated plate which comprises maintaining the temperature of the coated plate at around 100 C. without pressure, then raising the temperature gradually to about 190 C. while pressure is being applied against the surface of the selenium.

3. The method of processing a selenium coated plate which comprises maintaining the temperature of the coated plate at approximately 100 C. without pressure for about a few minutes, then raising the temperature gradually to approximately 190 C. over a period of approximately 15 minutes, and applying pressure during said temthe pressure is increased during the period of the heating.

5. The process according to claim 3 in which the initial pressure is about 240' pounds persquare inch and is increased during the heating to about 720 pounds per square inch.

6. The method of processing a selenium coated plate which comprises maintaining the plate at a temperature between the approximate limits of C. and 120 C. for about a few minutes without pressure and then raising the temperature to a temperature within about 160 C. to 218 C. with substantial pressure applied ag st the s face of the selenium.

7. The method according to claim 6 in which the substantial pressure is within the limits of about 250 pounds per square inch and 750 pounds per square inch.

8. The method of processing a selenium coated plate which comprises heating the coated plate at a temperature of around C. for five minutes without pressure, then applying a pressure of about 250 pounds per square inch for about a half minute during which time the temperature is raised from about 100 C. to about C., then raising the pressure to about 500 pounds per square inch for .the next half minute durin plate which comprises heating the plate at a temperature of about C. for a few minutes without pressure, then raising the temperature to about C. in about one and a half minutes without pressure, then applying about 240 pounds per square inch pressure for the next one and a half minutes during which time the temperature is raised to about C., then increasing the pressure to about 720 pounds per square inch for the next minute during which time the temperature is raised to about C., then continuing to raise the temperature under the 720 pounds per square inch pressure until the end of about sixteen minutes from the beginning of the heat treatment, and then releasing the pressure and raising the temperature to about 210 C. for about fourteen minutes more.-

10. The method of making a selenium plate for rectifiers and the like which comprises heat treating a metal plate coated with amorphous selenium at a temperature at which crystal nuclei are formed until a substantial number of such nuclei is produced and a substantial proportion of the selenium remains amorphous, then raising the temperature to a value only slightly below the melting point of selenium, applying pressure 7 6 to the selenium and continuing said heat and pressure until the selenium is substantially transformed to the crystalline form throughout.

11. The method set forth in claim 10 in which pressure is applied to the selenium during treatment at the lower temperature.

12. The method of making a selenium plate for rectifiers and the like which comprises heat treating a metal plate coated with amorphous selenium at a temperature in the neighborhood of 100 C. until a substantial number of nuclei is formed and a substantial proportion of the selenium remains amorphous, in the absence of any pressure sufiicient to produce a flow 0f the selenium, raising the temperature prior to completing crystallization to about C., applying pressure to the selenium and continuing the latter heat and pressure until the selenium is substantially transformed to the crystalline form.

131 The method of making a. selenium plate for rectifiers and the like which comprises heat treating a metal plate covered with amorphous selenium at a temperature at which crystal nuclei are formed until a substantial number of such nuclei is produced and a substantial proportion of the selenium remains amorphous, applying pressure to the selenium during said heat treating, then raising the temperature to a value only slightly below the melting point of selenium, applying higher pressure to the selenium and continuing said heat and higher pressure until the selenium is substantially transformed to the crystalline form throughout.

ARTHUR voN HIPPEL. 

